This invention relates generally to optical materials and more particularly to a method of forming bodies of optically transparent yttrium oxide.
As known in the art, there is a need for materials which are highly durable, and which have substantial optical transparency in both the visible and infrared optical bands. Applications for these materials include commercial systems such as metal vapor lamps and optical windows, as well as, military systems, such as airborne optical imaging systems. Optical imaging systems such as those found on an infrared heat seeking missile, generally have one or more optical elements such as windows or domes which are mounted on the external portion of the missile. These external elements are provided to isolate the remaining optics of the imaging system from the external environment through which the missile is flown. These external elements, therefore, must have a particularly high degree of resistance to environmental exposures and must have sufficient strength to protect the remaining components in the imaging system during operation of the imaging system in addition to having the aforementioned substantial transparency in the visible and/or infrared spectrums.
Several materials have been identified as potential candidates for these applications. Each of these materials generally has a high degree of strength and is theoretically capable of having a relatively high degree of infrared transparency, particularly within the wavelength range of approximately 2 microns to 5 microns. One material which is suitable in particular is yttrium oxide (Y.sub.2 O.sub.3).
In addition to the aforementioned optical requirement for optical transparency in the wavelength range of approximately 2 microns to 5 microns, it is also desirable in certain applications that the optical element be transparent to wavelengths beyond 5 microns. For example, in missiles which are designed to travel at very high speeds for long periods of time, a dome mounted on the front portion of such a missile will reach elevated temperatures. A "hot" dome made of materials which do not transmit to long wavelengths beyond 5 microns, for example, may produce undesirable emissions resulting in increased system noise in the optical imaging system shielded by the dome.
Moreover, associated with such a "hot" dome is a requirement that the material of the dome withstand the thermal induced stresses created from the aerodynamic heating of the surface thereof.
Some techniques for producing Y.sub.2 O.sub.3 domes involve sintering to substantially full density a Y.sub.2 O.sub.3 body. In these techniques, sintering aids such as La are added up to 10% by weight to achieve the high density. The addition of these sintering aids has one drawback. The sintering aids in a material such as Y.sub.2 O.sub.3 will reduce its thermal conductivity. Thus, reduced thermal conductivity will provide a concominant reduction in thermal shock resistance of a dome. A hot dome, therefore, is susceptible to damage due to the reduction in thermal shock resistance.
A technique has been described for producing high quality Y.sub.2 O.sub.3 which is suitable for the aforementioned applications. As described in U.S. Pat. No. 4,761,390 Hartnett et al., and assigned to the assignee of the present invention, relatively thick bodies of Y.sub.2 O.sub.3 are produced by densifying a consolidated body of substantially pure Y.sub.2 O.sub.3 by sintering at an elevated temperature until the body has a closed porosity density and then subjecting the closed porosity body to an elevated temperature and a simultaneous elevated isostatic pressure until final densification (approximately 99.99% of theoretical) has been achieved. The final densification step is followed by an annealing step to restore the body to clear transparency. The clear body has an optical transparency of greater than 73% over the wavelength range of 2-5 microns for samples having thickness up to 0.375 inches. The body also has a high thermal conductivity and relative small grain size and low coefficient of absorption. Thus, the material is a very desireable material for the above applications.
One drawback, as discussed in the patent, is that during the final densification step at the elevated temperature and elevated isostatic pressure, the yttria body is exposed to a reducing environment. Thus oxygen is lost from the body and the samples become non-stoichiometric. This oxygen defficiency results in a dark or black cast to the material. Therefore, in the above patent, a post-densification annealing step was performed to restore O.sub.2 stoichiometry, thus removing the dark appearance after the final densification step.
However, the samples occasionally have a slight yellow tinge after annealling which degrades the visible transmittance properties. Y.sub.2 O.sub.3 when subjected to a strongly oxidizing atmosphere gains oxygen interstitials. It is possible that these interstitials could cause this degradation in transmittance. Alternatively, it is possible that tungsten (W) contamination from the element of the vacuum furnace used during the sintering step could be present in the sample. This in turn would oxidize during annealling and form WO.sub.3, which is yellow.
It would be desirable, therefore, to eliminate the post densification annealling step to prevent yellow tinge of the samples, while at the same time providing clear, transparent Y.sub.2 O.sub.3 and otherwise maintain all of the advantageous optical and material properties described in the above-mentioned patent.